The present invention is in the field of battery technology and, more particularly, in the area of additives for use with high-energy electrolytes and electrodes in electrochemical cells.
A liquid electrolyte serves to transport ions between electrodes in a battery. Organic carbonate-based electrolytes are most commonly used in lithium-ion (“Li-ion”) batteries and, more recently, efforts have been made to develop new classes of electrolytes based on sulfones, silanes, and nitriles. Unfortunately, these conventional electrolytes typically cannot be operated at high voltages, since they are unstable above 4.2 V or other high voltages. At high voltages, conventional electrolytes can decompose, for example, by catalytic oxidation in the presence of cathode materials, to produce undesirable products that affect both the performance and safety of a battery. Conventional electrolytes may also be degraded by reduction by the electrodes when the cells are charged.
Solvents, salts, or additives have been incorporated into the electrolyte to decompose on the electrode to form a protective film called a solid electrolyte interphase (SEI). Depending on the exact chemical system, this film can be composed of organic or inorganic lithium salts, organic molecules, oligomers, or polymers. Often, several components of the electrolyte are involved in the formation of the SEI (e.g., lithium salt, solvent, and additives). As a result, depending on the rate of decomposition of the different components, the SEI can be more or less homogenous.
For high-energy cathode materials, electrolyte stability remains a challenge. Recently, the need for higher performance and high capacity lithium ion secondary batteries used for power sources is dramatically increasing. Lithium transition metal oxides such as LiCoO2 (“LCO”) and LiNi0.33Mn0.33Co0.33O2 (“NMC”) are state-of-the-art high-energy cathode materials used in commercial batteries. Yet only about 50% of the theoretical capacity of LCO or NMC cathodes can be used with stable cycle life. To obtain the higher capacity, batteries containing these high-energy materials need to be operated at higher voltages, such as voltages up to about 4.7V. However, above about 4.2V, conventional electrolytes degrade and this leads to a significant deterioration of the cycle life. Further, the decomposition of the electrolyte at higher voltages can generate gas (such as CO2, O2, ethylene, H2) and acidic products, both of which can damage a battery. These effects are further enhanced in “high nickel” NMC compositions such as LiNi0.6Mn0.2Co0.2O2 or LiNi0.8Mn0.1Co0.1O2 or others, which can provide higher capacities, due to the electrochemical nature of the nickel.
Many of these same challenges occur when a battery is operated at high temperature. That is, conventional electrolytes can be decomposed by oxidation or may be degraded by reduction at high temperature analogous to the way these mechanisms affect the electrolytes at high voltage. Other parasitic reactions can also occur at elevated temperature.
Lithium ion batteries operating at higher voltage, such as greater than 4.25V, are needed in order to meet increasing energy density requirements for automobile applications. However, the high voltage conditions can result in shortened cycle life and safety concerns, such as an increased risk of fire. Current state of the art electrolytes are known to be unstable at high voltages, especially at elevated temperatures. Electrolyte formulations currently used in most commercial lithium-ion batteries are alkyl carbonate based electrolytes, using LiPF6 as a salt. Formation of acidic species from thermal decomposition of LiPF6, as well as oxidative electrolyte decomposition at higher voltages, results in transition metal dissolution from the cathode surface. The resulting metal ions migrate to the anode where they are reduced on the anode surface, causing anode SEI decomposition and/or reformation. The SEI decomposition and/or reformation can then lead to increased lithium ion consumption as well as significant growth in the electrical impedance of the cell, both of which are undesirable.
Current electrolyte formulations that use LiPF6 can reach a high specific conductivity, about 10 mS cm−1 at room temperature, for example. These electrolyte formulations are typically able to passivate an aluminum current collector at the cathode and thereby prevent aluminum corrosion at high voltages. However, in presence of lithiated graphite and/or delithiated transition metal oxides, LiPF6 has limited thermal stability. This limited thermal stability negatively affects the usefulness of current electrolyte formulations in large-scale lithium ion batteries for electric vehicle (EV) applications. It is well known that LiPF6 will undergo thermal decomposition reactions to generate the strong Lewis acids, such as PF5 or even protic acid HF if even trace amounts of water are present. The resulting acidic species often trigger a number of undesirable chemical reactions, such as ring-opening reactions of cyclic carbonates and acid-base reactions with transition metal oxides in the cathode. These undesirable reactions often result in continuous consumption of electrolyte as well as lithium ions, which ultimately leads to capacity fade and impedance growth during cycling, especially cycling at elevated temperatures.
As disclosed herein, these challenges and others are addressed in high energy lithium-ion batteries operation at high voltage and at high temperature.